1. Field of the Invention
One embodiment of the disclosed invention relates to a method for forming a negative electrode and a method for manufacturing a lithium secondary battery.
2. Description of the Related Art
In recent years, portable devices such as smartphones and portable game consoles have become widespread. Further, with growing interest in environmental issues, hybrid cars and electric cars have attracted attention; thus, secondary batteries typified by lithium secondary batteries have gained in importance.
The basic structure of a secondary battery includes a positive electrode, a negative electrode, and an electrolyte (an electrolyte solution or a solid electrolyte) provided therebetween. Typically, the positive electrode and the negative electrode each include a current collector and an active material provided over the current collector. In the case of a lithium secondary battery, a material that can occlude and release lithium ions is used as an active material for a positive electrode and a negative electrode.
To increase the contact area between an active material and an electrolyte, particulate active materials are preferably used. Thus, a binder, a conductive additive, and the like are mixed with the particulate active materials to form an active material layer, and the active material layer is provided over a current collector to form an electrode (positive electrode or negative electrode) in some cases.
As a negative electrode active material, a material that can occlude and release ions serving as carriers (hereinafter referred to as carrier ions), such as carbon or silicon, is used. For example, silicon can occlude approximately ten times as many carrier ions as carbon and thus has higher theoretical capacity and is advantageous in increasing the capacity of the lithium secondary battery.
However, as the amount of occluded carrier ions is increased, the volume of an active material greatly changes with occlusion and release of the carrier ions in a charge and discharge cycle, resulting in lower adhesion between a current collector and silicon and deterioration in battery characteristics due to charge and discharge. Thus, a layer formed of silicon is formed over the current collector and a layer formed of graphite is formed over the layer formed of silicon, whereby deterioration in battery characteristics due to expansion and contraction of the layer formed of silicon is reduced (see Patent Document 1).
Since silicon has lower conductivity than carbon, the surfaces of silicon particles are coated with graphite and an active material layer including the silicon particles is formed over a current collector, whereby a negative electrode including the active material layer with increased conductivity is formed.
On the other hand, the use of graphene as a conductive electronic material in semiconductor devices has been studied recently. Graphene refers to a one-atom-thick sheet of carbon molecules having π bonds.
Graphene is chemically stable and has favorable electric characteristics and thus has been expected to be applied to channel regions of transistors, vias, wirings, and the like included in the semiconductor devices. In addition, to improve the conductivity of an electrode material for a lithium secondary battery, particulate active materials are coated with graphite or graphene (see Non-Patent Document 1).